A Distributed Antenna System (DAS) consists of antennas (or antenna arrays) spatially distributed in a geographical location and connected to a central processing unit (CPU) via backhaul. The CPU coordinates transmissions to all user equipments (UEs) in the geographical location via the distributed antennas. It has been shown that a DAS can achieve high user and sum throughputs in the downlink by broadcasting information to several UEs in the service area. This is described in a paper by G. Caire and S. Shamai, “On the achievable throughput of a multiantenna Gaussian broadcast channel,” published in the IEEE Trans. Info. Theory, vol. 49, no. 7, pp. 1691-1706, July 2003, and incorporated herein by reference.
Information broadcast as described in the above-referenced paper requires reasonably accurate knowledge of the propagation channels between all distributed antennas and all UEs involved in the broadcast. In a time division duplex (TDD) system, this knowledge can be acquired from the uplink channels. However, in a frequency division duplex (FDD) system, this knowledge has to be fed back from the UEs to the base antennas via feedback channels, which taxes the system capacity. Another problem is that information broadcast requires signal transmission from distributed antennas to be highly coherent. The scheme is not very robust against channel estimation errors, feedback delay and system synchronization errors.